Monolithic substrates serve as the active surface in a variety of exhaust gas treatment devices. Optionally impregnated with catalysts, the substrates comprise the active surfaces in catalytic converters, diesel particulate filters, selective catalyst reduction units. NOx traps, and other exhaust gas treatment devices.
In general, the operating temperature for a substrate is substantially higher than ambient temperatures; high enough that most conventional materials suffer adverse effects from the temperature sufficient to make them unacceptable candidates from which to make a substrate. The materials comprising monolithic substrates are commonly frangible or brittle materials exhibiting a high heal resistance, a low thermal expansion coefficient, and a low impact resistance. Without limitation, a common material type which is an acceptable candidate from which to make a monolithic substrate is ceramic, although metallic substrates are sometimes used.
The geometry comprising substrates typically promotes a high surface area to volume ratio. In certain embodiments, the substrate geometry comprises a plurality of elements which are thin and fragile. Without limitation, a common geometry for substrates is a monolith comprising an array of hollow rectangular prism cells defining tiny flow channels, separated by thin, fragile walls, such as in a honeycomb-type configuration.
Together, the geometric and material considerations for substrates commonly result in a substrate which is susceptible to impact, crushing, or other mechanical failure from small shockloads or stress, and which operates at very high temperatures. To address the problem of the fragile nature of the substrate, it is common to protect the substrate within a housing, typically a metallic housing with a space or gap between the external surface of the substrate and the internal surface of the housing. In order to protect the substrate from thermal and mechanical shock and other stresses, as well as to provide thermal insulation, it is known to position at least one sheet of mounting material within the gap between the substrate and the housing.
Because exhaust gas treatment devices are designed to operate at temperatures substantially higher than ambient temperatures and are designed to cool to ambient temperatures when not operating, exhaust gas treatment devices are designed to undergo significant temperature fluctuations. The mounting of the substrate is designed to protect the substrate over the entire scope of temperatures to which the device is exposed: from ambient through operating temperatures. The temperature fluctuations present a considerable challenge in designing the substrate mounting system.
Direct mounting of the substrate to the housing is possible but uncommon. Direct mounting is uncommon in part because the changes in temperature between operation cycles induce differing thermal changes in component size due to coefficient of thermal expansion differences for the substrate and the metal housing, sufficient to induce undesirable changes in mounting or holding forces. Absent a means to compensate for these differences, the mounting forces can change to levels insufficient to prevent undesirable vibration, shock, impact, or other motion. Another reason that direct mounting is uncommon is that heat from the substrate readily propagates to the housing under such mounting conditions. The resultant heating of the housing can result in the housing reaching undesirably high temperatures.
A more common means of mounting the substrate comprises inclusion of an insulating mounting mat between the substrate and the metallic housing. The mounting mat may be wrapped about the substrate and may be compressed by enclosing the housing around it. The level of compression is selected to provide an engagement force between the housing and the mat; and, the mat and the substrate, which produces mounting or holding forces both sufficiently high to secure the substrate with respect to the housing, and sufficiently low to avoid damage to the substrate. Also, a mounting mat will inherently have some resistance to heat flow and, in certain embodiments is a good insulator; the mat resists propagation of heat from the substrate to the housing and thereby lowers the steady state operating temperature of the housing for a given steady state operating temperature of the substrate.
Selection of a type of mounting material and the ambient temperature compression load to which to subject the mounting material to yield acceptable mounting or holding forces at all temperatures that the exhaust gas treatment device experiences continues to be a source of difficulty. Compounding this difficulty is the need for an insulative material between the substrate and the housing having a low thermal conductivity but which will not add undesirable weight or bulk to the device.